The present invention is broadly applicable to the production of alkylated hydrocarbons. These compounds are useful in themselves and more frequently in subsequent chemical synthesis of other compounds. The present invention is particularly applicable to the production of cumene, or isopropylbenzene. Another application of the present invention is in the production of ethylbenzene.
Many industrial alkylation processes use a Friedel-Crafts type of catalyst such as aluminum chloride or solid phosphoric acid (SPA). These catalysts are corrosive and may be difficult to dispose of. By contrast, zeolite catalysts are less corrosive and not as difficult to dispose of, and so zeolite catalysts have replaced Friedel-Crafts type catalysts in some commercial processes. Zeolite catalysts are also advantageous because generally they catalyze the alkylation of aromatics at lower reaction temperatures than Friedel-Crafts catalysts. Although this characteristic is usually an advantage from a catalytic viewpoint, it can also be a disadvantage from an energy-utilization viewpoint. This is because a lower reaction temperature generally also means that the temperature of the reactor effluent is lower too, and therefore the options for using the energy of the effluent elsewhere in the process are more limited. This is a specific illustration of a more general characteristic of energy utilization, namely that it is usually more difficult to utilize the "low grade" or waste heat in a stream that is at a relatively low temperature than it is to utilize the "high grade" heat in a stream at relatively high temperature.
In alkylation processes, for example, the "high grade" heat in the reactor effluent of prior art processes that use Friedel-Crafts catalysts is generally and most conveniently used in the rectifier to separate some of the unreacted aromatic from the effluent for recycle to the reactor U.S. Pat. No. 4,051,191 issued to Ward discloses such a process. In contrast, the heat in the reactor effluent of processes that use zeolite catalysts is "low grade" and generally is insufficient to recover much of the unreacted aromatic unless more energy is supplied to either the effluent itself or the depropanizer. For this reason, the waste heat of the effluent of related art processes that use zeolite catalysts is used in low temperature heating applications, such as preheating cold feed to the alkylation reactor. U.S. Pat. NO. 4,008,290 issued to Ward discloses such a process in which alkylation reactor feed is heated by recycling a portion of the alkylation effluent. Alternatively, the waste heat is used in waste heat boilers to produce low pressure steam. However, since it is common generally in hydrocarbon processing and particularly in alkylation processes that there is an abundance of such low pressure steam already, it is more efficient and economical to use this waste heat in some other useful way.
This invention utilizes the heat of reaction in a more effective manner than the prior art processes that use a depropanizing zone, such as the process disclosed in U.S. Pat. No. 4,587,370. In such prior art processes, the bottoms stream from the depropanizing zone passes into a benzene column which removes benzene that is recycled to the alkylation reactor. Since the benzene column generally operates at a lower pressure than the depropanizing zone which feeds it, the depropanizing zone bottoms stream generally passes through a pressure control valve before entering the benzene column. Despite this pressure decrease, a relatively small portion of the depropanizing zone bottoms stream vaporizes, and a substantial portion of the depropanizing zone bottoms stream, including benzene and mono- and poly-alkylated aromatic hydrocarbons, remain in the liquid phase. In the prior art processes, the temperature decrease, if any, that occurs when the depropanizing zone bottoms stream passes through the pressure control valve is not used for recovering heat to help separate the hydrocarbons in the stream.